This invention lies in the field of digital communications systems.
More particularly, it concerns a device which can be added to an analog radio communication system such as a citizen's band radio, for example, so that a selected station can be called once the transmitting and called stations have the apparatus of this invention.
Still more particularly, it is concerned with sending out from one transmitting station a coded digital signal representing the called station, and providing in the called station apparatus for decoding this transmission and detecting when the decoded signal matches the number of the called station.
In radio communication systems such as the citizen's band radios, for example, if an operator at a first station wishes to communicate with a second station, there is no commonly used way of calling that second station except to schedule a particular time and date of communication, so that the called station operator can hear the spoken word calling for a response from him.
Other than by prearranged timing, there is no common way for a first station to communicate specifically with a second station such as when there is a matter of urgency, for example. By the use of the method and apparatus of this invention, it is possible for a first station to key-in a selected group of decimal digits or binary bits, representative of the called station, and to cause a coded facsimile of this word to be transmitted over the air. Upon receipt by the called station, this coded train of pulses is decoded and converted to a binary word representative of the called station. This word is compared internally with a binary word which is the number of the called station. When the two numbers agree, an alarm signal is generated.